Powder coatings are dry, fine particles which are solid at room temperature and which, over recent years, have gained considerable acceptance over liquid coatings as surface finishes for a number of different types of substrates. Powder coatings are more environmentally friendly than liquid coatings because they are virtually free of harmful fugitive organic solvent carriers that are customarily present in liquid based coatings. This reduces or altogether eliminates solvent emission problems associated with air pollution and health risks experienced by the workers employed in either preparing or applying the coating material.
Early uses of powder coatings involved application onto metal substrates. Since these substrates can withstand the high temperatures that were required to fuse and cure these first generation powder coatings, application was limited to these types of substrates. However, the technology has evolved to the point where powder coatings are now being employed to coat heat sensitive materials, such as wood, fiberboard and plastics which, due to the sensitive nature of the substrates, require that the powder coating be capable of fusing (as in the case of thermoplastic coatings) or fusing and curing (as in the case of thermoset coatings) at comparatively low temperatures. Low temperature curable coatings reduce or altogether eliminate charring or warping of the substrate.
A common issue which must be addressed when coating wood or fiberboard is that of moisture escaping from the substrate during the fusing/curing cycle(s). This is referred to as “outgassing”. These substrates contain entrapped moisture, typically between about 3 and about 10 percent by weight. This moisture is not altogether undesirable, however. Moisture is helpful as an element in the electrostatic application of coating powder in that it enables the otherwise non-conductive cellulosic material to hold sufficient electrical charge for the efficient electrostatic application of powder coatings. However, uncontrolled outgassing from the substrate when heat is applied during the fusing and/or curing cycles must be addressed.
In a wood substrate of generally uniform density, outgassing is usually evenly distributed throughout the entire surface of the coated object. However, in medium to high density fiberboard, different regions of the board material will inherently have different densities which will contain different levels of entrapped moisture. Medium density fiberboard (MDF), which is most often used to manufacture doors and panels for office furniture and kitchen cabinets, is a very porous substrate which contains water and other volatiles.
During the process of applying a powder coating to MDF substrates, it is customary to preheat the substrate prior to powder application in order to release some of the entrapped moisture and to aid in the application of the powder coating to the target surface. However, this step provides inconsistent results since moisture content can vary significantly due to variations in the manufacture of the board, storage conditions and coating application techniques. After the application of the powder coating surface finish, these moisture content variations will result in surface blisters, pinholes or inconsistent surface appearance spots, often called “dry spots”.
Certain types of powder coatings may experience an increased number of problems related to surface blemishes. For example, the LAMINEER® coatings sold by the Morton Powder Coatings subsidiary of the Rohm and Haas Company seem particularly susceptible to outgassing related problems. Some solutions have been attempted, such as by increasing the pre-heat temperatures. While this process modification tends to reduce blistering, it is an incomplete fix for the appearance of dry spots. Further, if pre-heat temperatures are increased too high or the pre-heat cycle is maintained for too long a period of time, the MDF substrate will lose so much moisture that the electrostatic charge necessary for the application of powder coatings will be all but eliminated, thus inhibiting the successful application of a uniform powder coating finish.
MDF is manufactured by the digestion and milling of cellulosic materials which are provided by wood, sawdust, certain grasses and even cereal grain byproducts. Resins are then added as binders and the blended composition is then formed into a mat and compressed into sheets by either continuous line or multi-opening presses in the presence of heat to cure the binder. This process produces MDF boards which inherently possess a density gradient through its cross section. “Typical” MDF boards exhibit a density that is higher at its surface than at its core. Density may also vary from one surface to the other, opposite surface. These density variances will result in differences in the rate and amount of outgassing for each lot of board material.
One approach to eliminating outgassing defects and enabling the production of a uniform surface finish is disclosed in U.S. Pat. No. 6,136,370. This patent discloses the process of providing grooves or holes of various depths in the “back” or non-cosmetic side of the MDF panels, a process referred to as “back-routing”. This process works very well with MDF parts where the back side is hidden from view and the grooves or holes are never visible in the final, assembled part. The problem with this approach, however, is that where both sides of a panel are visible either all the time or at various times during the operating of the piece of furniture or cabinet of which the MDF panel is a part, both surfaces must be as aesthetically pleasing as possible. This precludes the employment of the technique of the '370 patent. An example of such types of panels are what are referred to as “Flipper Doors” used in overhead office storage cabinets.
The present invention addresses the need to control the outgassing of moisture and other volatiles from within MDF substrates during the application of powder coatings. This is achieved by methods which will provide an aesthetically blemish-free front appearance surface with a minimum of blemishes on the opposing or back appearance surface of MDF panels. These methods will now be described in detail.
It is known that, with the equal application of heat to both surfaces of MDF, moisture and other volatiles will tend to escape, or outgas, out the less dense surface first. If both sides are coated with equal thicknesses of powder coating and heated evenly, outgassing will occur more on the less dense surface, leaving pin-holes, dry spots and other blemishes. The more dense surface will exhibit a relatively blemish free finish.